1. Field of the Invention
The present invention relates to power supply and, more particularly, to a power supply suitable for display devices.
2. Description of the Related Art
Conventionally, various types of discharge-type display devices have been developed such as plasma display panels (hereinafter referred to as PDPS). It is necessary to supply at least two types of voltages to the discharge-type display device. The two types of voltages are a drive voltage of several tens to several ten thousands of volts for turning on the display elements and a control voltage of several volts employed as the power supply voltage for a control circuit of the display elements.
In the conventional power supply which outputs such different voltages at the same time, when the power supply output is shut down, the drive voltage is first disabled and then the control voltage is disabled in sequence. This sequence is required for the following reason. If the control voltage is first disabled prior to the drive voltage at the time of shutting down the power supply output, a high voltage would be applied to control and display ICs. This would present problems of causing the ICs to be damaged or unnecessary images to appear on the display device.
In Japanese Patent Laid-Open Publication No.Hei 4-91624, disclosed is a conventional power supply which outputs such different voltages at the same time (a first conventional example). FIG. 1 is a circuit block diagram illustrating the configuration of the power supply of the first prior art. A power supply 109 shown in FIG. 1 includes a rectifier circuit 102 connected to an external commercial power source 100; switches 104a, 104b disposed between the commercial power source 100 and the rectifier circuit 102; a capacitor 105 connected to the output of the rectifier circuit 102; and a DC/DC converter 101, connected to the commercial power source 100 via the rectifier circuit 102, for generating output voltages of +5V and +35V. The power supply 109 also includes an output terminal 106a, connected to the output of the DC/DC converter 101, for providing an output voltage of +5V; an output terminal 106b, connected to the output of the DC/DC converter 101, for providing an output voltage of +35V; an optical coupling isolator 103 for outputting a signal in accordance with the input voltage of the DC/DC converter 101; and a switching circuit 107 connected to the output of the optical coupling isolator 103 and including a photo MOS relay 108 which is opened or closed in accordance with a signal outputted from the optical coupling isolator 103. The switching circuit 107 is connected between the DC/DC converter 101 and the output terminal 106b to open or close the connection between the DC/DC converter 101 and the output terminal 106b. 
In the power supply of the first prior art, the optical coupling isolator 103 detects the input voltage of the DC/DC converter 101. Shutting down the power supply output from the commercial power source 100 will cause the optical coupling isolator 103 to output a signal to the photo MOS relay 108. This will cause the photo MOS relay 108 to be opened forcedly, the switching circuit 107 to be opened, and the voltage of 35V to be shut down which needs to be disabled quickly. This provides the aforementioned sequence of disabling first the drive voltage and then disabling the control voltage.
In Japanese Patent Laid-Open Publication No.Hei 7-104711, disclosed is a power supply for an LCD (Liquid Crystal Display) (second prior art). FIG. 2 is a circuit block diagram illustrating the configuration of the LCD incorporating the power supply of the second prior art. The LCD shown in FIG. 2 includes batteries 211, a DC/DC converter 214 for converting the output voltage of the batteries 211 to a control voltage Vcc, and a power switching transistor 212 connected between the batteries 211 and the DC/DC converter 214. The LCD also includes a power control IC 213 connected to the base of the power switching transistor 212 via a resistor R204; a timer 215 connected to the power control IC 213; an inverter 216 connected to the timer 215; and an AND gate 217, the input of which is connected to the inverter 216 and a delay device 201A and the output of which is connected to the base of an LCD power front-stage transistor 219 via a resistor R205. The LCD also includes an LCD power rear-stage transistor 220, the collector of which is connected to the power switching transistor 212 and the base of which is connected to the collector of the LCD power front-stage transistor 219 via a resistor R206; a device logic portion 201E to which the control voltage Vcc is supplied from the DC/DC converter 214: and a DC/DC converter 218 for converting the output voltage of the batteries 211 to a drive voltage Vee. The LCD further includes an LCD panel portion 201F to which the drive voltage Vee is supplied from the DC/DC converter 218; a delay device 201A which is connected between the output terminal of the DC/DC converter 214 and the ground and which includes a resistor and a capacitor; and an FET 201D driven by the delay device 201A. The device logic portion 201E is the control circuit of the LCD panel portion 201F. In addition, the LCD has a capacitor C201 connected to the DC/DC converter 218. Furthermore, the LCD is provided with a resistor R208 connected to the higher potential side of the FET 201D in series therewith. Still furthermore, a resistor R207 is connected between the collector of the LCD power front-stage transistor 219 and the FET 201D.
In the power supply of the second prior art, when the power from the batteries 211 is shut down, the delay device 201A causes the FET 201D to conduct. This allows the drive voltage Vee to be disabled forcedly prior to the control voltage Vcc, thereby providing the aforementioned sequencing.
However, each of the aforementioned techniques present the problems described below. The power supply of the first prior art has a problem of requiring the employment of the optical coupling isolator 103 and the photo MOS relay 108, thereby leading to an increase in cost of the power supply.
On the other hand, the power supply of the second prior art is a power circuit for an LCD display device and therefore supplies a drive voltage of several tens of volts. In contrast, a drive voltage of several hundreds to several ten thousands of volts is required for discharge-type display devices such as CRTs (Cathode Ray Tubes) or PDPs. In the power supply of the second prior art, such high voltages would be produced to cause the following problems.
FIG. 3 is a graph illustrating the time dependency of the output voltage of the power supply of the second prior art. As shown in FIG. 3, the drive voltage Vee and the control voltage Vcc increase together after the power output has been turned on and decrease together after the power has been turned off. At this time, the power supply of the second prior art allows the control voltage Vcc to be disabled in a short period of time. On the contrary, as shown in FIG. 2, the power supply has the capacitor C201 of a large-capacity connected to the output terminal of the DC/DC converter 218. This will not allow the power accumulated in the capacitor C201 to be discharged quickly, causing the drive voltage Vee to be sustained at a high voltage for a long period of time. This causes the device logic portion 201E to stop the operation thereof first, whereas a high voltage is kept being applied to the LCD panel portion 201F. This causes the drive voltage Vee of a high voltage to be applied to the drive circuit of the inactivated LCD panel portion 201F and to control circuits such as the gate circuit of the control IC of the device logic portion 201E. When a high voltage is generated in the power supply of the second prior art to drive a discharge-type display device, the drive voltage Vee as high as several hundreds to several ten thousands of volts would be applied to the drive and control circuits of the display device to cause damage to these circuits in some cases.
Furthermore, a discharge circuit would be required to discharge the power accumulated in the capacitor C201 when the power supply output is shut down. A discharge current of high voltage and high current would pass through the resistor R208 and the FET 201D of the second prior art, which constitute the discharge circuit. This makes it necessary to provide an extremely high allowable voltage for these components. This causes a problem of significantly increasing the cost for manufacturing the power supply.
It is an object of the present invention to provide a power supply which has a simple configuration and which is fabricated at a low cost without components of a high allowable voltage. It is another object of the present invention to provide a power supply which can provide an extended period of time for the voltage output from the low voltage power circuit and thereby ensure the power shut-down sequence of disabling the drive voltage prior to the control voltage, whereby the drive and control circuits of the display device can be prevented from being damaged by the application of the drive voltage.
A power supply according to the present invention comprises a first power circuit, supplied with a voltage from an external power source, for outputting a first voltage; a first capacitor for being charged with said first voltage; and a second power circuit, supplied with a voltage from said external power source, for outputting a second voltage lower than said first voltage. The power supply also comprises a voltage converter circuit, an input terminal thereof being connected to a high voltage side of said first capacitor, and an output terminal thereof being connected to an output terminal of said second power circuit, wherein power accumulated in said first capacitor is converted to a voltage to be supplied to the output terminal of said second power circuit when said external power source stops supplying the voltage.
In the power supply according to the present invention, the power accumulated in the first capacitor is discharged to the output of the second power circuit via the voltage converter circuit after the power supply output has been shut down. Upon shutting down the power supply output, this allows the first voltage to be disabled quickly in a short period of time and sustain the second voltage for a fixed period of time. This makes it possible to disable the drive voltage prior to the control voltage in a discharge-type display device incorporating the power supply according to the present invention when the first voltage is employed as the drive voltage for driving the display device and the second voltage is employed as the control voltage for controlling the display device. This prevents a high drive voltage from being applied to the control and drive circuits of the display device. This makes it possible to prevent damage to the control and drive circuits of the display device.
Furthermore, it is not necessary to provide components such as an optical coupling isolator or photo MOS relay for the power supply according to the present invention. Furthermore, the voltage converter circuit converts the power accumulated in the first capacitor to the second voltage for supply, thereby obviating the need of a discharge circuit of a large capacity which is essential to the conventional power supply. This makes it possible to provide the power supply at a reduced cost.
Another power supply according to the present invention comprises a first power circuit, supplied with a voltage from an external power source, for outputting a first voltage; a first capacitor for being charged with said first voltage; and a second power circuit, supplied with a voltage from said external power source, for outputting a second voltage lower than said first voltage. The power supply according to the present invention also comprises a voltage converter circuit, an input terminal thereof being connected to a high voltage side of said first capacitor, and an output terminal thereof being connected to an input terminal of said second power circuit, wherein power accumulated in said first capacitor is converted to a voltage to be supplied to the input terminal of said second power circuit when said external power source stops supplying the voltage.
After the power supply output has been shut down, the present invention allows the power accumulated in the first capacitor to be discharged to the input of the second power circuit via the voltage converter circuit, thereby making it possible to further stabilize the second voltage.